1. Field of the Invention
The present invention relates to a method of forming a polycrystalline semiconductor film, and a semiconductor device and a display apparatus fabricated using the method. More particularly, the present invention relates to a method of forming a polycrystalline semiconductor film, which has a small amount of crystal defects, on a non-single crystal insulating film or a non-single crystal insulating substrate. The polycrystalline semiconductor film is produced by applying heat energy and light energy (strong irradiation) to an amorphous semiconductor film. And the present invention relates to a semiconductor device, such as a liquid crystal driver, a semiconductor memory, a semiconductor logic circuit, and the like, comprising a polycrystalline semiconductor film formed by the method. And the present invention relates to a display apparatus comprising the semiconductor device.
2. Description of the Related Art
Conventionally, there has been a known method of crystallizing an amorphous semiconductor film provided on a non-single crystal insulating film or substrate by applying energy.
An example of such a method is disclosed in TECHNICAL REPORT OF IEICE (the Institute of Electronics, Information and Communication Engineers), Vol. 100, No. 2, ED2000-14 (April, 2000) pp. 27-32 (hereinafter referred to as conventional example 1). Specifically, PE-CVD (Plasma Enhanced Chemical Vapor Deposition) is used to form an amorphous silicon film having a thickness of 45-50 nm on a glass substrate, and thereafter, the amorphous silicon film is irradiated with excimer laser light, so that the film is crystallized into a polycrystalline silicon film having a grain size of 700 nm. In conventional example 1, when a polycrystalline silicon film obtained by this method was used to fabricate a thin film transistor (TFT), the mobility was improved up to 320 cm2/Vxc2x7sec.
A crystallization method is disclosed in Japanese Laid-Open Publication No. 2000-150382 (hereinafter referred to as conventional example 2). Specifically, a catalytic substance is introduced into a surface of an amorphous silicon film, the resultant amorphous silicon film is crystallized by thermal treatment, followed by irradiation with laser light, whereby a crystalline silicon film having improved crystallinity can be obtained.
FIG. 7 is a schematic diagram for explaining the crystallization method described in conventional example 2.
In the method of conventional example 2, an amorphous silicon film 2 having a thickness of 100 nm is formed on a glass substrate 1 using PE-CVD, and thereafter, a silicon oxide film 3 having a thickness of about 2 nm is formed on the amorphous silicon film 2 in order to improve wettability.
Thereafter, a solution containing nickel, which is a catalytic substance for accelerating crystallization, is applied onto the silicon oxide film 3, followed by spinning and drying, so that a solution film 4 is formed on the silicon oxide film 3.
Thereafter, in this situation, annealing is performed at 550xc2x0 C. for 4 hours to crystallize the amorphous silicon film 2.
Thereafter, the crystallized silicon film 2 is irradiated with KrF excimer laser light having a wavelength of 248 nm and an energy density of 200-350 mJ/cm2 to improve the crystallinity.
In such a crystallization method of conventional example 2, since crystallization is accelerated by a catalytic substance, a crystalline silicon film can be obtained at low temperature in a short time.
However, the method of conventional example 1 has the following drawback. The irradiation of an amorphous silicon film with laser light is not optimized, so that crystal grains having a small diameter of several micrometers are obtained, potentially leading to a polycrystalline silicon film containing a number of grain boundaries. The grain boundary acts as a recombination center which provides a trap level for carriers. Therefore, when a TFT is fabricated using a polycrystal containing a number of grain boundaries, the mobility of the TFT is reduced.
The method of conventional example 1 also has the following drawback. Since it is not easy to irradiate the entire surface of a large-area substrate uniformly with sufficiently stable laser light, it is difficult to form a silicon film having uniform crystallinity.
The method of conventional example 2 has the following drawback. In the method, the silicon film 2 crystallized with the introduced catalytic substance is irradiated with laser light so as to improve the crystallinity. The optimum conditions for the laser light irradiation are not disclosed. A number of crystal defects may occur in a silicon film formed by the method.
If such a semiconductor film having a number of crystal defects is used to fabricate a semiconductor device (transistor), such as a liquid crystal driver, a semiconductor memory, a semiconductor logic circuit, and the like, problems arise, such as the mobility of carriers is small, the threshold voltage is high, and the like. Moreover, dispersions in carrier mobility and threshold voltage are large between a number of semiconductor devices (transistors) fabricated in a liquid crystal driver, or the like.
According to an aspect of the present invention, a semiconductor film is provided, which comprises a polycrystalline semiconductor film provided on a substrate having an insulating surface. Nearly all crystal orientation angle differences between adjacent crystal grains constituting the polycrystalline semiconductor film are present in the ranges of less than 10xc2x0 or 58xc2x0-62xc2x0.
In one embodiment of this invention, the proportion of the crystal orientation angle differences between adjacent crystal grains of 1xc2x0-10xc2x0 or 58xc2x0-62xc2x0 is 0.5-1.
In one embodiment of this invention, the polycrystalline semiconductor film is made of silicon.
According to another aspect of the present invention, a method of forming a semiconductor film is provided, which comprises the steps of: forming an amorphous semiconductor film on a substrate having an insulating surface; introducing a catalytic substance for accelerating crystallization into a surface of the amorphous semiconductor film; applying first energy to the amorphous semiconductor film to crystallize the amorphous semiconductor film into a crystalline semiconductor film; and applying second energy to the crystalline semiconductor film so that nearly all crystal orientation angle differences between adjacent crystal grains are present in the ranges of less than 10xc2x0 or 58xc2x0-62xc2x0. The crystallinity of the crystalline semiconductor film is increased to be turned into a polycrystalline semiconductor film.
In one embodiment of this invention, the first energy is heat energy and the second energy is strong light.
In one embodiment of this invention, the energy density of the strong light is such that after irradiation of the strong light, the proportion of the crystal orientation angle difference between adjacent crystal grains of less than 10xc2x0 or 58xc2x0-62xc2x0 is highest.
In one embodiment of this invention, the semiconductor film is made of silicon.
In one embodiment of this invention, the catalytic substance is a metal selected from the group consisting of Fe, Co, Ni, Cu, Ge, Pd, and Au, a compound containing at least one of these metals, or a combination of at least one of these metals and a compound containing at least one of these metals.
In one embodiment of this invention, the concentration of the catalytic substance at a surface of the amorphous semiconductor film is greater than or equal to 1xc3x971011 atoms/cm2 and smaller than or equal to 1xc3x971016 atoms/cm2.
In one embodiment of this invention, the strong light is excimer laser light.
According to another aspect of the present invention, a semiconductor device is provided, which comprises the above-described semiconductor film.
According to another aspect of the present invention, a display apparatus is provided, which comprises the above-described semiconductor device.
Thus, the invention described herein makes possible the advantages of providing a method of forming a semiconductor film having a reduced number of crystal defects and good crystallinity, and a semiconductor device and a display apparatus fabricated by the method.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.